The present invention relates to a silicon wafer in which oxygen precipitation is stably obtained regardless of device production process and position in crystal and a method for producing it, as well as a method for evaluating defect regions of a silicon wafer of which pulling conditions are unknown.
In recent years, in connection with the use of finer devices accompanying the use of higher integration degree of semiconductor circuits such as DRAM, demand for quality of silicon single crystals produced by the Czochralski method (it may be also abbreviated as xe2x80x9cCZ methodxe2x80x9d hereinafter) from which substrates therefor are produced is becoming higher. In particular, since there are defect called grown-in defects such as FPD, LSTD and COP and they degrade device characteristics, reduction of these defects is considered important.
Prior to explanation of those defects, there will be given first general knowledge of factors determining densities of defects introduced into silicon single crystals, a void type point defect called vacancy (also abbreviated as V hereinafter), and an interstitial type silicon point defect called interstitial silicon (interstitial-Si, also abbreviated as I hereinafter).
A V-region in a silicon single crystal means a region containing many vacancies, i.e., depressions, holes and so forth generated due to shortage of silicon atoms, and an I-region means a region containing many dislocations and aggregations of excessive silicon atoms generated due to excessive amount of silicon atoms. Between the V-region and the I-region, there should be a neutral region (also abbreviated as N-region hereinafter) with no (or little) shortage or no (or little) surplus of the atoms. Further, it has become clear that the aforementioned grown-in defects (FPD, LSTD, COP etc.) should be generated strictly only with supersaturated V or I, and they would not be present as defects even though there is little unevenness of atoms so long as V or I is not saturated.
It is known that densities of these two kinds of point defects are determined by the relationship between the crystal pulling rate (growing rate), and the temperature gradient G in the vicinity of the solid-liquid interface in the crystal in the CZ method. It has also been confirmed that defects distributed in a ring shape called OSF (Oxidation Induced Stacking Fault) are present in the N-region between the V-region and the I-region. Since OSFs are generated in a shape of concentric circle observed in a wafer surface when the wafer is sliced from a single crystal, there is used a term of OSF ring.
Those defects generated during the crystal growth are classified as follows. For example, when the growth rate is relatively high, i.e., around 0.6 mm/min or higher, grown-in defects considered to be originated from voids, i.e., aggregations of vacancy-type point defects, such as FPD, LSTD and COP, are distributed over the entire cross-section of the crystal along the radial direction at a high density, and a region containing such defects is called V-rich region (region in which supersaturated vacancies form void defects). When the growth rate is 0.6 mm/min or lower, with the decrease of the growth rate, the aforementioned OSF ring is initially generated at the circumferential part of the crystal, and L/D (large dislocations, abbreviation of interstitial dislocation loops, which include LSEPD, LFPD and so forth), which are considered to be originated from dislocation loops, are present outside the ring at a low density. A region containing such defects is called I-rich region (region in which supersaturated interstitial silicons form dislocation loop defects). When the growth rate is further lowered to around 0.4 mm/min or lower, the OSF ring shrinks and disappears at the center of wafer, and thus the entire plane becomes the I-rich region.
Recently, there has been discovered a region called N-region between the V-rich region and the I-rich region, and outside the OSF ring, in which neither of the void-originated FPD, LSTD and COP, the dislocation loop-originated LSEPD and LFPD and OSF are present. This region exists outside the OSF ring, and shows substantially no oxygen precipitation when it is subjected to a heat treatment for oxygen precipitation and examined by X-ray analysis or the like as for the precipitation contrast. This region is present at rather I-rich side, and the interstitial silicon point defects are not so rich as to form LSEPD and LFPD.
Presence of the N-region was also confirmed inside the OSF ring, in which neither of void-originated defects, dislocation loop-originated defects and OSFs were present.
Because these N-regions are formed obliquely with respect to the growing axis when the growth rate is lowered in a conventional growing method, it exists as only a part of the wafer plane.
As for this N-region, according to the Voronkov""s theory (V. V. Voronkov, Journal of Crystal Growth, 59 (1982) 625-643), it was proposed that a parameter of F/G, which is a ratio of the pulling rate (F) and the crystal solid-liquid interface temperature gradient (G) along the growing axis, determined the total density of the point defects. In view of this, because the pulling rate should be constant in a plane, for example, a crystal having a V-rich region at the center, I-rich region at the periphery, and N-region between them is inevitably obtained at a certain pulling rate due to distribution of G in the plane.
Therefore, improvement of such distribution of G has recently been attempted, and it has become possible to produce a crystal having the N-region spreading over an entire transverse plane of the crystal, which region could previously exist only obliquely, for example, at a certain pulling rate when the crystal is pulled with a gradually decreasing pulling rate F. Further, such an N-region spreading over an entire transverse plane can be made larger to some extent along the longitudinal direction of the crystal by pulling the crystal at a pulling rate maintained at the value at which the N-region transversely spreads. Furthermore, it has also become possible to make the N-region spreading over the entire transverse plane somewhat larger along the growing direction by controlling the pulling rate considering the variation of G with the crystal growth to compensate it, so that the F/G should strictly be maintained constant.
As further classification of the N-region, it is known that there are NV-region (region in which there are many vacancies, but void defects are not detected), which is present outside the OSF ring and adjacent to it, and NI-region (region in which there are many interstitial silicons but dislocation loop defects are not detected), which is adjacent to the I-rich region.
Furthermore, in a silicon substrate produced by the CZ method, control of oxygen precipitation is becoming increasingly important in view of internal gettering effect against heavy metal impurities in addition to the importance of the reduction of such grown-in defects. However, since the oxygen precipitation strongly depends on the heat treatment conditions, it is a very difficult problem to obtain suitable oxygen precipitation in the device production process, which may be different for every user. Furthermore, wafers are subjected to a heat treatment not only in the device production step, but also a heat treatment in the crystal pulling step, in which the temperature is changed from the melting point to room temperature (thermal history of crystal). Therefore, in an as-grown crystal, there already exist oxygen precipitation nuclei formed during the thermal history of the crystal (grown-in precipitation nuclei). Such presence of grown-in precipitation nuclei makes the control of oxygen precipitation still difficult.
The oxygen precipitation process in the device production process can be classified into two kinds of processes. One is a process in which grown-in precipitation nuclei that remained after the initial heat treatment of the device production step grow, and the other one is a process in which nuclei generated during the device production step grow. In the latter case, since the oxygen precipitation strongly depends on oxygen concentration, it can be controlled by controlling the oxygen concentration. On the other hand, in the former case, thermal stability of grown-in precipitation nuclei (i.e., at how much degree of density they can remain at the temperature of the initial stage of the process) constitutes an important point.
For example, even if the grown-in precipitation nuclei exist at a high density, if they have a small size, they become thermally unstable and disappear during the initial heat treatment of the device production process. Thus, oxygen precipitation cannot be secured. The problem in this case is that, since the thermal stability of grown-in precipitation nuclei strongly depends on the thermal history of crystal, oxygen precipitation behavior may markedly vary in the device production step depending on the crystal pulling conditions or position in the crystal along the crystal axis even for wafers having the same initial oxygen concentration. Therefore, in order to control the oxygen precipitation in the device production step, it is important to control not only the oxygen concentration but also the thermal stability of grown-in precipitation nuclei by controlling the thermal history of crystal.
Although development of techniques for reducing the aforementioned grown-in defects is currently proceeding, the thermal history of low defect crystals produced by such techniques is controlled in order to reduce grown-in defects. It is considered that this also changes the thermal stability of grown-in precipitation nuclei. However, it is not known at all how much degree it is changed.
Therefore, it is expected that the oxygen precipitation behavior in such low defect crystals may significantly vary in the device production step, and it results in reduction of device yield.
Further, since no method for judging from which defect region a wafer is produced has been established as for a wafer of which defect regions are unknown, it is difficult to predict oxygen precipitation behavior in the wafer during the device production step.
Therefore, the present invention was accomplished in view of the aforementioned problems, and its object is to provide a silicon wafer that stably provides oxygen precipitation regardless of position in crystal or device production process, and a method for producing it. Another object of the present invention is to provide a method for evaluating defect regions of a silicon wafer of which pulling conditions are unknown and thus of which defect regions are also unknown.
The present invention was accomplished in order to achieve the aforementioned objects, and provides a silicon wafer having an NV-region, an NV-region containing an OSF ring region or an OSF ring region for the entire plane of the silicon wafer and having an interstitial oxygen concentration of 14 ppma (Japan Electronic Industry Development Association (JEIDA) Standard) or less.
If an entire plane of silicon wafer consists of a NV-region, OSF ring region or both of them as described above, there would be an appropriate amount of thermally stable large grown-in precipitation nuclei, therefore fluctuation of oxygen precipitation becomes little even if the device process may be different, and BMDs (oxide precipitates called Bulk Micro Defects) can be stably obtained. Further, if an interstitial oxygen concentration is 14 ppma or less, density of small grown-in precipitation nuclei becomes low, and therefore the wafer can be a silicon wafer showing reduced fluctuation of oxide precipitates depending on the position in crystal.
The present invention also provides a silicon wafer obtained by slicing a silicon single crystal ingot grown by the Czochralski method with nitrogen doping, wherein the silicon wafer has an NV-region, an NV-region containing an OSF ring region or an OSF ring region for its entire plane.
If a silicon wafer is doped with nitrogen and has a NV-region, OSF ring region or both of them for the entire plane as described above, thermally stable large grown-in precipitation nuclei will be obtained at a high density, and therefore it becomes a silicon wafer providing sufficient gettering effect in the device production process.
In this case, nitrogen concentration doped in the silicon wafer is 1xc3x971010 to 5xc3x971015 number/cm3.
That is, the above range was defined because at least a nitrogen concentration of 1xc3x971010 number/cm3 or more is necessary in order to obtain BMDs at an extremely high density by the effect of nitrogen doping, and a concentration of more than 5xc3x971015 number/cm3 may inhibit single crystallization during the pulling of a single crystal ingot by the CZ method.
Further, also in the case of a wafer doped with nitrogen, if the interstitial oxygen concentration is 14 ppma or less, the density of small grown-in precipitation nuclei become low, and hence the fluctuation of oxide precipitates depending on the position in crystal can be reduced.
The present invention also provides a method for producing a silicon wafer, wherein, when a silicon single crystal is grown by the Czochralski method, the crystal is pulled with such conditions as present in an NV region or an OSF ring region in a defect distribution chart showing defect distribution which is plotted with D [mm] in the horizontal axis and F/G [mm2/xc2x0 C.xc2x7min] in the vertical axis, wherein D represents a distance between center of the crystal and periphery of the crystal, F [mm/min] represents a pulling rate and G [xc2x0 C./mm] represents an average temperature gradient in the crystal along the crystal pulling axis direction in the temperature range of from the melting point of silicon to 1400xc2x0 C., so that interstitial oxygen concentration should become 14 ppma or less.
If a crystal is pulled while the pulling rate F of the crystal and the average temperature gradient G in the crystal along the crystal pulling axis direction in the temperature range of from the melting point of silicon to 1400xc2x0 C. are controlled so that the conditions should be present in a region defined by a boundary between a V-rich region and an NV-region and a boundary between an NV-region and an NI-rich region in the defect distribution chart shown in FIG. 8, which was obtained through analysis of the results of experiments and researches, as described above, a silicon wafer obtained by slicing the grown single crystal ingot can have an NV-region, NV-region containing an OSF ring region or an OSF ring region for its entire plane, and at the same time, the crystal can be pulled so that an interstitial oxygen concentration should become 14 ppma or less.
In such a region, there would be an appropriate amount of thermally stable large grown-in precipitation nuclei. Therefore, fluctuation of oxygen precipitation becomes little even if the device process may be different, and BMDs can be stably obtained. Further, because the interstitial oxygen concentration is 14 ppma or less, density of small grown-in precipitation nuclei becomes low, and therefore the fluctuation of oxide precipitates depending on the position in crystal can be reduced.
The present invention also provides a method for producing a silicon wafer, wherein, when a silicon single crystal is grown by the Czochralski method, the crystal is pulled with such conditions as present in an NV region or an OSF ring region in a defect distribution chart showing defect distribution which is plotted with D [mm] in the horizontal axis and F/G [mm2/xc2x0 C.xc2x7min] in the vertical axis, wherein D represents a distance between center of the crystal and periphery of the crystal, F [mm/min] represents a pulling rate and G [xc2x0 C./mm] represents an average temperature gradient in the crystal along the crystal pulling axis direction in the temperature range of from the melting point of silicon to 1400xc2x0 C., and with nitrogen doping.
If a crystal is pulled with such conditions as described above, a silicon wafer obtained by slicing the grown single crystal ingot can be doped with nitrogen and have an NV-region, an NV-region containing an OSF ring region or an OSF ring region for its entire plane.
If a wafer is doped with nitrogen and has an NV-region, OSF ring region or both of them for its entire plane as described above, thermally stable large grown-in precipitation nuclei can be obtained at a high density, and therefore a wafer that can provide sufficient gettering effect in the device production process can be produced.
In this case, nitrogen concentration to be doped can be 1xc3x971010 to 5xc3x971015 number/cm3.
Further, also in this case, when a crystal is grown by the CZ method, the crystal can be pulled so that the interstitial oxygen concentration should become 14 ppma or less.
In order to obtain BMDs in an extremely high density by the effect of nitrogen doping as described above, a nitrogen concentration of 1xc3x971010 number/cm3 or more is necessary. But if the concentration exceeds 5xc3x971015 number/cm3, the single crystallization may be inhibited during the pulling of a single crystal ingot by the CZ method. Therefore, a concentration of 5xc3x971015 number/cm3 or less is preferred.
Further, even when nitrogen is doped, if the interstitial oxygen concentration is 14 ppma or less, the density of small grown-in precipitation nuclei becomes low. Therefore, the fluctuation of oxide precipitates depending on the position in crystal can be reduced.
The present invention further provides a method for evaluating defect regions of a silicon wafer produced by the CZ method, wherein a defect region of a silicon wafer to be evaluated is evaluated by comparing at least two of oxide precipitate densities measured by the following steps:
(1) a wafer to be evaluated is divided into two or more pieces (A, B, . . . ),
(2) Wafer piece A among the divided pieces is loaded into a heat treatment furnace maintained at a temperature of T1 [xc2x0 C.] selected from a temperature range of 600-900xc2x0 C.,
(3) the temperature is increased from T1 [xc2x0 C.] to a temperature of 1000xc2x0 C. or higher, T2 [xc2x0 C.], at a temperature increasing rate of t [xc2x0 C./min] (provided that t is 3xc2x0 C./min or less), and the temperature is maintained until oxide precipitates in Wafer piece A grow to have a detectable size,
(4) Wafer piece A is unloaded from the heat treatment furnace, and oxide precipitates density in the wafer piece is measured,
(5) another wafer piece among the divided wafer pieces, Wafer piece B, is loaded into a heat treatment furnace maintained at a temperature of T3 [xc2x00 C.] selected from a temperature range of 800-1100xc2x0 C. (provided that T1 less than T3 less than T2),
(6) the temperature is increased from T3 [xc2x0 C.] to the temperature of T2 [xc2x0 C.] at the temperature increasing rate of t [xc2x0 C./min], and the temperature is maintained until oxide precipitates in Wafer piece B grow to have a detectable size, and
(7) Wafer piece B is unloaded from the heat treatment furnace, and oxide precipitates density in the wafer piece is measured.
As for a wafer of which defect regions are unknown, no method for judging from which defect region the wafer is produced has hitherto been established. Therefore, it has been difficult to predict oxygen precipitation behavior of the wafer in the device production step. However, by the aforementioned method for evaluating defect regions, defect regions of wafer of which pulling conditions are unknown and thus of which defect regions are also unknown can be evaluated, and at the same time, it becomes possible to predict oxygen precipitation behavior of the wafer in the device production step.
As described above, according to the present invention, stable oxygen precipitation can be obtained regardless of the position in crystal or the device production process, and therefore a wafer showing little fluctuation of oxide precipitate density and stable gettering ability can be obtained. Furthermore, by using the evaluation method of the present invention, defect regions of wafer of which pulling conditions are unknown and thus of which defect regions are also unknown can be judged relatively easily.